One of the handcrafted oak pieces in this puzzle (the larger square) is thinner than all the others and will slide out through a slot in the side wall ... if you can maneuver it into the corner position. It is permitted to slide th pieces about, but not to remove, lift or turn them.
Diagrams of 12 starting positions are provided with the puzzle, listed in order of difficulty according to the fewest number of moves in which it is possible to solve the problem. Puzzle 1 has 43 moves (at best) while it takes 263 to solve challenge number 12.
Solutions are not included, but are available by mail order from the manufacturer. Good luck !